Non-contact power collection systems such as used in moving vehicle systems generally employ one of two approaches: inductive pickups or linear synchronous motors. The inductive pickup has been used in electrified autobuses. It is essentially an iron-core transformer with the primary winding buried in the highway and the secondary winding on the autobus. Large amounts of iron are required on both the primary and the secondary windings in order to obtain a magnetic circuit with sufficiently low reluctance. Just as in a transformer, an alternating current in the primary induces a current in the secondary which can be used to drive an electric motor.
The second approach, linear motors, are a linear version of the more common rotary motors. In ground transportation applications, the stator may be mounted on the guideway. In its simplest embodiment, the stator is a series of coils wound so that a traveling magnetic field is induced in the stator when alternating current is driven through the coils. For most linear motors, the stator has an iron core with slots for the windings, which increases the flux linkage with the magnetic elements on the vehicle. The linear synchronous motor consists of a stator with a traveling magnetic field on the guideway, as described above, plus a permanent magnet which is fixed to the vehicle. Variable frequency wayside power is used in order to control the speed of the traveling magnetic field. When the vehicle is stationary, direct current (DC) is used in the coils to establish a magnet pole just ahead of the vehicle magnet. As the vehicle is pulled forward by the magnetic force, the current in the coils is shifted forward. As the vehicle picks up speed, the position of the magnetic poles in the stator is controlled by the frequency of the current in the coils to stay just ahead of the vehicle magnet, thus maintaining positive thrust over a range of speeds.
With the linear synchronous motor, not only is variable frequency wayside power required, but the guideway must be divided into a number of short segments or "blocks", with a separate variable frequency power control for each block. Only one vehicle can be on a block at a time. The exact phase relationship between the magnet field and the position of the vehicle must be sensed by the vehicle and transmitted back to the wayside controls in order to maintain synchronous operation. The linear synchronous motor actually bypasses the problem of current collection. A propulsive thrust is produced, but no electrical power is generated on the vehicle.